Theory of HFV
High Frequency Ventilation
• Defined by FDA as a ventilator that delivers more than
150 breaths/min.
• Delivers a small tidal volume, usually less than or equal
to anatomical dead space volume.
• While HFV’s are frequently described by their delivery
method, they are usually classified by their exhalation
mechanism (active or passive).
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Differences between HFOV and CMV
CMV
HFOV
Rates
0 - 150
180 - 900
Tidal Volume
4 - 20 ml/kg
0.1 - 5 ml/kg
Alv Press
0 - > 50 cmH2O
0.1 - 5 cmH2O
End Exp Vol
Low
Normalized
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HFV Gas Exchange
•
•
Henderson first published his findings in 1915, assessing
dead space relationship in ventilation.
He stated, “there may easily be a gaseous exchange
sufficient to support life even when Vt is considerably
less than dead space.”
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HFV Gas Exchange
•
•
In the 1970’s, Bunnell and his associates demonstrated
in animals that adequate alveolar ventilation could be
achieved with a frequency between 5 - 30 Hz and a Vt of
20 - 25% less volume than anatomical dead space.
Slutsky, et al. theorized that the gas exchange
mechanism was caused by the “coupled effects” of
convection and molecular diffusion.
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HFV Gas Exchange
•
Chang theorized that convective processes were more
predominant with an increase in Vt and lower
frequencies. A diffusive mechanism may be more
predominant where there is a decrease in Vt and higher
frequencies are used.
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High Frequency Ventilation
• Types of HFV’s Approved for use in both Neonates and Pediatrics
• SensorMedics 3100A
HFOV
• Bird Volumetric Diffusive
HFPPV
• Types of HFV’s Approved for use in Neonates Only
• Bunnell Life Pulse
HFJV
• Infrasonics Infant Star (discontinued)
HFFI
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Bunnell Life Pulse Jet
• Delivers a pulse of gas
into the ETT via a special
adapter and pinch valve
mechanism
• Exhalation is Passive
• Frequency of 4 - 11 Hz
• Peak Airway Pressure of
8- 50 cmH2O
• Used in tandem with a
conventional ventilator
• Mean Airway Pressure
limited to conventional
ventilator capabilities
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Infrasonics Infant Star HFFI
• Modification of the
conventional Infant Star
• Facilitated/Passive
Exhalation
• Pressure waveform
manipulated by a series of
pneumatic valves
• Frequency of 2 - 22 Hz
• Paw cannot be adjusted
directly. Usually adjusted by
changing end expiratory
pressure on CMV (limited to
24 cmH2O)
• Fixed 18 ms inspiratory time
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Bird Volumetric Diffusive Vent
• A pneumatic cartridge
(Phasitron) interrupts
the pressurized gas
source
• Passive Exhalation
• Frequency of 1.6 - 21.6
Hz
• Paw is not directly
adjusted
• May deliver HFV on top
of a conventional
breath
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Sensor Medics 3100A
• Electrically powered,
electronically controlled
piston-diaphragm
oscillator
• Paw of 3 - 45 cmH2O
• Pressure Amplitude from
8 - 110 cmH2O
• Frequency of 3 - 15 Hz
• Inspiratory Time 30% - 50%
• Flow rates from 0 - 40
LPM
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3100A
• “True” Oscillator
• Produces an active exhalation, and does not depend
on passive recoil of the chest for CO2 removal
• Stand Alone Ventilator
• Does not require nor deliver a conventional breath
through the system
• Does not require a special ET tube
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Volume delivery and MAP
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Comparison: Volume delivery
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3100A Ventilator
•Approved in 1991 for
Neonatal Application for the
treatment of all forms of
respiratory failure.
•Approved in 1995 for
Pediatric Application, with no
upper “weight limit”. For
treating selected patients
failing conventional ventilation.
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Remember
• Each device has specific labeling restrictions for its use.
Many devices are classified as “RESCUE ONLY” or for
treating only a specific pathology.
• No clinical comparisons between devices.
• Each has its own indications and risks.
• Strategies to treat each disease process is specific to
the device used.
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